Light output devices utilizing light emitting diodes (LEDs) as their light sources have become increasingly popular. Such light output devices can be used for illumination of objects, for display of an image, or simply for decorative purposes. Applications are for example decorative lighting for shelves, showcases, facades, office partitions, wall claddings.
LEDs are made by embedding LED chips in a package, for example a resin. This allows an improved manufacturability for light output devices utilizing LEDs. Such an embedded LED is also called a LED package.
It is known per se to embed a LED package in a thermoplastic material. The thermoplastic material embedding an LED package therein can be sandwiched between two glass plates to obtain a device called LEDs-in-glass. It is also possible to have an application in which no sandwiching glasses are used. An example is a plastic window in which LED packages are embedded. The embedding of the LED package in a thermoplastic material makes the device more wear resistant. The thermoplastic material prevents LED packages from being damaged and increases electrical safety of the device by covering electrical contacts and wires. Suitable thermoplastic materials in view of manufacturability of the device are for example polyvinyl butyral (PVB) or UV resin.
In such an arrangement, there is a need for controlling the optical and thermal properties for various reasons. For example, LED packages used in light output devices act as point sources resulting in bright spots of light. In LEDs-in-glass, there is a further problem that the light may be captured between the two glass plates due to total internal reflection.
A further problem with a device as described in the introductory paragraph is that the maximum power and light output is limited by the heat generated by the LED chips in the LED packages. The maximum allowed power is especially limited due to the heat insulating properties of the thermoplastic material, resulting in the thermoplastic material obstructing the heat from being conducted away from the LED package. The LED packages may also cause ‘hot spots’ which may hurt a person while handling the light output device.
US 2007/0025108 A1 discloses a circuit for a lighting installation, which comprises a thermoplastic resin base sheet having a structure in which a light emitting diode device is received in an illuminating position. This resin base sheet contains particles for increasing the heat conductivity. No solution is disclosed for the problem of bright spots.